Revert "Temporarily disable miri on doctests"

This reverts commit 3bfab6e.

Author: dtolnay

.github/workflows/ci.yml

@@ -154,7 +154,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@miri
       - run: cargo miri setup
-      - run: cd serde && cargo miri test --features derive,rc,unstable --all-targets  # exclude doctests https://github.com/rust-lang/miri/issues/3404
+      - run: cd serde && cargo miri test --features derive,rc,unstable
         env:
           MIRIFLAGS: -Zmiri-strict-provenance
       - run: cd test_suite && cargo miri test --features unstable